This invention relates to apparatus for locating discontinuities in an optical fiber waveguide or cable and, in particular, to means for coupling a light source and detector to the waveguide being tested.
The development of low-loss optical fibers has led to an increasing number of applications for light transmission systems. However, optical fibers are sometimes broken during manufacture, installation or use, and it is necessary to provide apparatus for determining the location of the break or fault accurately and quickly. Also, it is often desirable to determine the length of an unfaulted section of cable, the location of a splice or connector or the location of microcracks, bubbles or foreign inclusions sometimes produced by the manufacturing process. Accordingly, optical time domain reflectometers (OTDR) have been developed for this purpose.
In an OTDR, a high intensity light pulse, usually generated by a laser, is injected into an accessible end of the optical fiber under test. The light is transmitted through the fiber until it encounters a discontinuity such as a break, splice, connector or open end which causes some of the energy to be reflected back to the end at which the pulse was injected. The reflected energy is detected at the injection end of the fiber making it possible to determine the distance to the discontinuity. In addition to discrete reflections, there is a continuum of reflected energy called Rayleigh backscatter which can be used to measure the attenuation of the fiber and also the losses incurred at discontinuities.
One of the most critical and difficult design problems is to achieve a device which will efficiently couple light energy into one end of an optical fiber to be tested, couple energy reflected by a discontinuity in the test fiber back to a detector at the injection end of the fiber and, at the same time, prevent saturation of the detector by energy reflected from surfaces not located within the fiber under test.
More specifically, to achieve high sensitivity and resolution in an OTDR, a high intensity light source capable of generating a fast rising or short duration pulse is used in combination with a high gain, fast response photodetector. However, this combination of elements can lead to Fresnel reflection at the fiber input and to reflection of a relatively large portion of the initial light pulse energy from optical components coupling the light source and photodetector to the test fiber. While the coupling of some energy back to the photodetector from the interface with the optical fiber being tested is unavoidable (and may be used to advantage to mark the time t.sub.0 at which the initial pulse enters the fiber), the combined intensity of these reflections can be much greater than the intensity of the energy reflected from a discontinuity within the test fiber. Consequently, the photodetector may be saturated by the combined reflections causing it to become insensitive to the relatively low-level reflections from discontinuities in the test fiber. Further, under saturation conditions, an optical pulse having a duration of only a few nanoseconds can produce an output signal from the photodetector of several microseconds duration thereby tending to obscure pulses reflected by discontinuities close to the injection end of the fiber.
Various attempts have been made to solve this problem. In a paper titled "Photon-Probe--An Optical-Fiber Time-Domain Reflectometer", The Bell System Technical Journal, Vol. 56, No. 3, March 1977, Personick, there is disclosed an instrument that incorporates a gated photomultiplier receiver which is turned on immediately after an undesired echo has arrived thereby preventing detector saturation by strong nearby echoes. Another system described by Barnoski et al in an article "Optical Time Domain Reflectometer" published in Applied Optics, Vol. 16, No. 9, September 1977 employs a directional taper coupling to eliminate the need for either electrical or optical gating in preventing detector saturation. Still another system is described by Nelson et al in an unpublished paper "A Fiber-Optical Time Domain Reflectometer" wherein the detection of faults close to the injection end of a test fiber is attained by using, in conjunction with a polarized beam splitter, a semiconductor laser which emits light that is polarized more in one direction than another. The Personick and Nelson et al systems require beam splitters and all employ lenses making these prior art devices relatively cumbersome and complex.